1. Field of the Invention
The present invention relates to a next generation mobile communication system, and more particularly, to a data communication system transmitting data using adaptive modulation at an acknowledged data transfer mode simultaneously.
2. Discussion of the Related Art
Generally, a protocol for realizing a data communication system is divided into first, second, third, . . . and higher layers.
The first layer is a physical layer, in which encoding/decoding of data, rate matching, modulation/demodulation, spreading/dispreading and the like are defined.
The second layer includes two sub-layers such as a MAC(medium access control) layer and a RLC(radio link control) layer.
The MAC layer provides services such as data transfer, reporting of measurements(such as traffic volume, quality indication, etc.), and allocation/deallocation of radio resources.
And, the RLC layer provides services such as connection establishment/release of radio link, transparent data transfer, unacknowledged data transfer, acknowledged data transfer, QoS(quality of service) setting, notification of unrecoverable errors, and multicast delivery of higher layer messages.
And, the third layer includes a RRC(radio resource control) layer.
Data communication comes true by the services of the above-mentioned protocol layers.
Specifically, the unacknowledged data transfer and acknowledged data transfer of the services provided by the RLC layer of the second layer determine a transfer mode of data communication.
The acknowledged data mode guarantees the security of data transfer, and is ensured by means of retransmission.
In aspect of tendency of the latest standardization of data communication, widely is a method of varying a data rate by changing a coding type in accordance with radio environment so as to cope adaptively with the constantly varying radio environments. This is called adaptive modulation.
Yet, the related art has failed to propose a system realizing method considering both data transfer and adaptive modulation at the acknowledged data transfer simultaneously.
When information is transferred by applying adaptive modulation to a data communication system using the acknowledged data transfer mode, an effective transfer length is varied by the varying radio environments before one full information is completely transferred.
In this case, since the effective transfer length of data is varied, the system should form packets of new length. The system then retransmits the reformed packets.
In order to retransmit the reformed packets in the related art, the previously established radio link connection is released. After the packets for retransmission have been reformed, new radio link connection should be established. The release and establishment of the radio link are defined in the second layer.
In the related art, the radio link connection should be reestablished so as to guarantee the transmission security of single information, thereby bringing about inefficiency of transmission delay.
Besides, adaptive modulation should be applied in order not to establish the above radio link connection again. Nevertheless, the transmission delay of packets still occurs as well as the probability of error occurrence in the transmitted packets increases.